GPU: Add SDL_CopyGPUTextureToBuffer and SDL_CopyGPUBufferToTexture

[thatcosmonaut]

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Description

GPU-timelined texture-to-buffer and buffer-to-texture copies can be useful, so we should expose that capability.

I still need to investigate what expected behavior would be when copying depth data from depth-stencil textures.

D3D12's horrible design continues to be a delight. We'll need to figure out a workaround for the 256-byte alignment requirement.

Existing Issue(s)

Resolves #16248

TODO

• How do we want to handle stencil data in depth-stencil textures?
• Metal implementation
• NDA implementation
• Alignment workaround for D3D12

[thatcosmonaut]

From MSDN...

Note If you use CopyTextureRegion with a depth-stencil buffer or a multisampled resource, you must copy the entire subresource rectangle. In this situation, you must pass 0 to the DstX, DstY, and DstZ parameters and NULL to the pSrcBox parameter. In addition, source and destination resources, which are represented by the pSrcResource and pDstResource parameters, should have identical sample count values.

So you can't copy subregions of a depth texture (or a multisampled resource, though that's less useful in practice.) I guess we can validate this? It's a very annoying restriction.